Luminol chemiluminescence induced by silver nanoparticles in the presence of nucleophiles and Cu2+

JournalofPhotochemistryandPhotobiologyA:Chemistry215 (2010) 185–190

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JournalofPhotochemistryandPhotobiologyA:

Chemistry

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LuminolchemiluminescenceinducedbysilvernanoparticlesinthepresenceofnucleophilesandCu2+

NaLi,JieGu,HuaCui

CASKeyLaboratoryofSoftMatterChemistry,DepartmentofChemistry,UniversityofScienceandTechnologyofChina(USTC),230026Hefei,Anhui,PRChina

articleinfoabstract

Itwasfoundthatsilvernanoparticles(NPs)inthepresenceofnucleophilesandCu2+couldinduceluminoltoproducechemiluminescence(CL).TheAgNPs–NaBr–CuSO4–luminolsystemwaschosenasamodelsystemtostudytheCLmechanism.UV–visabsorptionspectrashowedthatsilvernanoparticlesinthepresenceofNaBrcouldreactwithCuSO4beforeinjectionofluminol.TheX-raydiffractionpat-ternsandX-rayphotoelectronspectrademonstratedthatCu(I)complexwasakeyreactionproductinAgNPs–NaBr–CuSO4system.Besides,itwasalsofoundthatsuperoxidedismutasecouldinhibittheCL,revealingthatasuperoxideanionwasinvolvedintheCLreaction.Onthisbasis,itwassuggestedthattheluminolCLinducedbysilvernanoparticlesinthepresenceofNaBrandCu2+derivedfromCu(I)complexformedviathereductionofCuSO4byAgNPsbytheaidofNaBr,whichreactedwiththedissolvedoxygentogeneratethesuperoxideanion;thenthesuperoxideanionreactedwithluminoltoproduceCL.Br asanucleophilewasconsideredtodecreasetheoxidationpotentialofsilvernanoparticlessothatCu(II)isreadilyreducedtoCu(I)andtobindtoCu(I)preventingCu(I)fromdismutationinwater.Asexpected,othernucleophilessuchasCl ,I andthiosulfate,whichwerealsoef cienttodecreasetheoxidationpotentialofAgNPsandbindtoCu(I),couldalsoinducetheluminolCL.

© 2010 Elsevier B.V. All rights reserved.

Articlehistory:

Received4May2010

Receivedinrevisedform30July2010Accepted17August2010

Available online 24 August 2010Keywords:

AgnanoparticleNucleophile

ChemiluminescenceLuminol

1.Introduction

Inrecentyears,nanoparticles(NPs)havebeenwidelystudiedfortheirexcellentproperties[1–3]andtheirpotentialapplicationsinmicroelectronics,optics,electronics,magneticdevices,andcataly-sis[4–8].Intheseinvestigations,anumberofreportsfocusonthepropertiesofNPsinthepresenceofnucleophiles[9–11].Absorp-tionofthesenucleophilesonsilverNPscandrasticallychangetheiropticalpropertiesandredoxreactivity[12,13].Thesilveratomsonthesurfacearecoordinativelyunsaturated.Onemayexpectthatunoccupiedorbitalsexistonthesurfaceintowhichanucleophilicreagentcandonateanelectronpair.Theconsequenceisnotonlyachangeintheopticalabsorptionofthecolloidalparticlesbutalsoachangeintheirreactivity.Theinteractionofsurfaceatomswithnucleophilesleadstoapartialoxidationofsurfaceatoms,whiletheinteriorofcolloidalparticlesreceivesacorrespondingnegativechargewhichcanbepickedupbytheelectronacceptors.Cu2+hasbeenusedasatypicalelectronacceptortotesttheenhancedredoxcapabilityofthesilverNPsinthenucleophiles.Withoutthenucle-ophiles,additionofCu2+couldnotresultintheoxidationofsilverparticles.TheFermileverofsilverNPsmodi edbynucleophileswassigni cantlyshiftedtowardanegativepotential,andismore

Correspondingauthor.Tel.:+865513606645;fax:+865513600730.E-mailaddress:hcui@(H.Cui).1010-6030/$–seefrontmatter© 2010 Elsevier B.V. All rights reserved.doi:10.1016/j.jphotochem.2010.08.016

negativethanthestandardredoxpotentialoftheCu2+/Cu+cou-ple.Asaresult,thereducingabilityofsilverNPsinthepresenceofnucleophilesisenhancedsothatCu2+couldoxidizesilverparticlesbytheaidofthenucleophiles.Besides,otherelectronacceptorshavebeeninvestigatedtotesttheenhancedredoxcapabilityofthesilverparticlesinthepresenceofnucleophiles.Thefactthattheoxidationofmetalbyoxygenisfacilitatedbyanucleophilicreagentiswell-known[9,10,13].CytochromecwasalsofoundtoundergoreductionwhenintroducedintoasuspensionofcolloidalsilvermixedwithaqueousBr andI inmicromolarconcentration[14].

Aclassofhomogeneouschemicalreactions,chemilumines-cence(CL)reactionsaccompaniedbylightemissions,havebeenintensivelyinvestigatedformanyyears.Recently,metalNPswereemployedintheCLstudyasacatalyst,reductant,luminophor,andenergyacceptor[15–27].However,NPsinthepresenceofnucle-ophileswiththeenhancedreducingabilitywererarelyinvestigatedforCLreactions.Inourpreviouswork,itwasfoundthatinthepres-enceofKI,silverNPsasareductantcouldinducetheliquid-phaseCLoflucigenin[18].Furthermore,theCLbehavioroflucigenin-Agcol-loidinthepresenceofothernucleophilessuchasthiourea,sodiumthiosulfate,cysteine,mercaptoaceticacid,andmercaptopropionicacidwasexplored.InsteadofAgNPs,AuandPtwerealsoexaminedforinducinglucigeninCLinthepresenceofvariousnucleophiles.Inthepresentwork,luminolCLreaction,anotherimportantCLreac-tion,inducedbysilverNPsinthepresenceofnucleophileswas

186N.Lietal./JournalofPhotochemistryandPhotobiologyA:Chemistry215 (2010) 185–190

explored.ItisfoundthatAgNPsinthepresenceofNaBrandCu2+couldreactwithluminol,accompanyingbyaCLemission.TheCLmechanismwasalsostudied.2.Experimental

2.1.Chemicalsandsolutions

A1.0×10 2mol/Lstocksolutionofluminolwaspreparedbydissolvingluminol(Sigma,America)in0.10mol/Lsodiumhydrox-idesolution.Polyvinylpyrrolidone(PVP,Mw=36,000),superoxidedismutase(SOD),AgNO3,NaBr,NaI,NaCl,thiosulfate,CuSO4,andNaBH4wereobtainedfromShanghaiReagent(Shanghai,China).Allreagentswereofanalyticalgrade,andusedasreceived.Ultra-purewaterobtainedbyaDirect-Q3UVwaterpuri cationsystem(Millipore,USA)wasusedthroughout.2.2.PreparationofAgcolloid

AgColloidwassynthesizedbychemicalreductionmethodinasolution.Inbrief,0.50gPVPwasdissolvedin20.0mLwater.Subsequently,25mLNaBH4(5.0×10 3mol/L)wasmixedwiththePVPaqueoussolution.Then,5.0mLAgNO3(5.0×10 3mol/L)wereaddeddropwisetothemixturewithvigorousstirringsimultane-ously.Thecolorofthemixturevisiblychangedtoyellowgradually.Thereactionwasconductedatroomtemperaturefor30minandagedfor2daysat4 Cbeforeuse.Therawcolloidwascentrifugedat12,500rpmfor30min(Universal320,Hettich,Germany).Agcolloidwasobtainedafterthesedimentwasredissolvedin1%PVPaqueoussolution.Theresultingyellowsilvercolloidswerecharacterizedbyhighresolutiontransmissionelectronmicroscope(HRTEM,JEM-2010,Hitachi,Japan).ThediametersandsizedistributionofAgNPswereobtainedbasedonthestatisticalanalysisofHRTEMdataasshowninFig.1.TheimageofFig.1showedtheAgNPswerewelldistributed.StatisticalanalysisofHRTEMdatarevealedthattheaveragediameteroftheNPswas8.0±4.0nm.2.3.CLmeasurements

TheCLwasdetectedbyamicroplateluminometer(CentroLB960,Berthold,Germany).Inatypicalexperiment,themixedsolu-tionof50 LofAgcolloidwith50 LofNaBrwaspipetted

into

Fig.1.HRTEMimagesofAgNPs.TheinsetisthecorrespondingsizedistributionsofAgNPs.Calculateddiameteris8.0±4.0nmbyaccountingabout100units.

eachwellofmicrotiterplate,andthen50 LofCuSO4and50 Lofluminolwereinjectedintothemixtureinthewellsuccessively.Thelightemissionwasmeasuredbythemicroplateluminometerimmediately.

2.4.Opticalmeasurements

TheCLspectraweremeasuredonamodelFL5401spectro u-orometer(Shimadzu,Japan).UV–visspectraweremeasuredonamodelUV-2401PCspectrophotometer(Shimadzu,Japan).ThereactionproductoftheAgcolloid–NaBr–CuSO4systemwascharac-terizedbyX-raypowderdiffraction(XRD)andX-rayphotoelectronspectrum(XPS).XRDpatternwasobtainedwithamodelD/max-rAdiffractometer(Rigaku,Japan)andXPSwasobtainedwithamodelESCALABMKIIelectronspectrograph(VG,England).Thesamplewaspreparedasfollows:4mLCuSO4(0.1mol/L)wasaddedinto400mLAgcolloidcontaining1.0×10 2mol/LNaBr.Thesolutionwascontinuouslystirredfor60min.Afterthereaction nishedcompletely,thereactedsystemwasevaporatedandprecipitatesoccurredinthereactedmixturegradually.Then,theobtainedpre-cipitateswere ltratedbyamixedcellulosemembranewithporesizeof0.45 m(XingYa,Shanghai).Subsequently,theprecipitateswerethoroughlywashedbywaterandethanolandthendriedatroomtemperatureforanight,whichwasusedforthefurtherexper-iments.

3.Resultsanddiscussion

3.1.CLaccompaniedbyreactionofluminol–CuSO4–AgNPsinthepresenceofNaBr

TheCLresponseofAgNPsinthepresenceofNaBrandCuSO4toalkalineluminolsolutionwasstudiedbystaticinjection.AsshowinFig.2, rst,whenAgcolloidwasmixedwithNaBr,noCLemis-sionappeared(thelinesbeforeinjection).Subsequently,CuSO4andluminolwereinjectedtothemixturesuccessively,arepro-ducibleCLwasobserved(curvea).ThemaximumCLintensityoftheCLsystemwasaboutoneorderofmagnitudehigherthanthatofaconventionalluminol–H2O2CLsystem(0.001mol/LH2O2)withthesamepHvalueoftheCLreactionandthesameconcentrationofluminol.ThesupernatantoftheAgcolloidaftercentrifugationwasusedasacontrolsolutionfortheCLexperimentsinsteadofAgcolloidandnoconsiderableCLemissionwasobserved

(curve

Fig.2.ChemiluminescencekineticcurveswheninjectingCuSO4andluminolintoAgcolloidinthepresenceofNaBr(a).Thesupernatantsolutionaftercentrifuga-tionofAgcolloidwasusedasacontrolsolution(b).Reactionconditions:CuSO4,1×10 3mol/L;luminol,1×10 4mol/Lin0.1mol/Lcarbonatebuffer(pH10.0);Ag,3×10 4mol/L;NaBr,1×10 2mol/L.Theinsetisthechemiluminescencespectra.

N.Lietal./JournalofPhotochemistryandPhotobiologyA:Chemistry215 (2010) 185–190

187

Fig.3.EffectsofthereactionconditionsontheCLintensity.(A)EffectofthepH,reactionconditions:1×10 3mol/LCuSO4,0.01mol/LNaBr,1×10 4mol/Lluminolin0.1mol/LcarbonatebufferswithvaryingpHvalues.(B)Effectoftheluminolconcentration,reactionconditions:pH10.0,1×10 3mol/LCuSO4,0.01mol/LNaBr,luminolwithdifferentconcentrationsin0.1mol/Lcarbonatebuffers.(C)EffectoftheCuSO4concentration,reactionconditions:pH10.3,0.01mol/LNaBr,1×10 4mol/Lluminolin0.1mol/Lcarbonatebuffers.(D)EffectoftheNaBrconcentration,reactionconditions:pH10.3,1×10 3mol/LCuSO4,1×10 4mol/Lluminolin0.1mol/Lcarbonatebuffers.

b).Therefore,theCLaccompanyingtheluminol–CuSO4–AgcolloidreactioninthepresenceofNaBrwasrelatedtotheAgNPsratherthanassociatedspeciespresentinthesynthesis.TheCLspectrashowedthatthemaximumemissionwavelengthwasca.425nm,indicatingthattheluminophorwas3-aminophthalate,anoxida-tionproductofluminol[28].

TheeffectsofthereactionconditionsontheCLintensitywereinvestigated.AsshowninFig.3,theCLintensitywaslargelydepen-dentonthepHofthereactionandthereactantconcentrations,includingCuSO4,NaBr,andluminol.TheCLintensityincreasedwiththepHfrom9.5to10.3,whichwassimilartootherluminolsystems,butitdecreasedwhenthepHwashigherthan10.3(Fig.3(A)).ItmaybeduetothatCuSO4withhighconcentrationsunderultra-basicconditionwouldformCu(OH)2precipitatesbeforetheCLreactions.IncreasingtheconcentrationofthehydroxideioncausesmoreCu(OH)2toprecipitate,whichdecreaseconcentrationofCu2+fromthesolution.BasedontheresultsinFig.3(C),thedecreaseofCu2+concentration(0.5–0mmol/L)leadstothedecreaseoftheCLintensity.Accordingly,thedecreaseoftheCu2+concentrationinthereactionmightbethereasonforthedecreaseoftheCLintensityduringthehighpHrange.AsshowninFig.3(B),theCLintensityincreasedwiththeluminolconcentrationovertherangeof1.0×10 7to1.0×10 3mol/L.AsshowninFig.3(C),theCLintensityincreasedobviouslywiththeCuSO4concentrationover0–0.5mmol/L,butitdecreasedslightlywhentheCuSO4con-centrationwashigherthan0.5mmol/L.TheeffectoftheNaBrconcentrationontheCLintensitywassimilarastheeffectoftheCuSO4concentrationasshownin3(D).WiththeNaBrconcen-trationover0–2.0mmol/L,theCLintensityincreased.However,whentheconcentrationwashigherthan2mmol/L,theCLintensitydecreasedgradually.ItisreasonablethattheincreaseofCLinten-sityfollowedbytheinitialincreaseoftheconcentrationofCuSO4orNaBrbecausetheyarereactantsintheCLreaction.However,furtherincreaseoftheconcentrationofCuSO4orNaBralsoledtoanincreaseinionicstrength.Itwasreportedthattheincreaseof

ionicstrengthcouldcausetheCLdecrease[29].Asaresult,theCLintensitydecreasedslightlyorremainedintactbeyondaparticularconcentrationofCuSO4orNaBr.3.2.MechanismoftheCLreaction

ItreportedthatCu2+couldoxidizeAgcolloidinthepresenceofnucleophiles[12].Therefore,itwasdeducedthatCuSO4mightreactwithAgNPsbytheaidofNaBratthebeginning;then,luminolmightreactwiththeintermediateproductsofAgNPs–NaBr–CuSO4toyieldaCLemissionwhenluminolwasinjectedinthemixture.Tovalidatethishypothesis,supportingexperimentswerecarriedoutasfollows.

Firstofall,UV–visabsorptionspectrawereutilizedtomon-itorthereactionbetweenCuSO4andAgNPsinthepresenceofNaBr.AgNPsexhibitedSPRabsorptionbandsinthevisibleregion(ca.410nm)beforethereaction,ingoodagreementwithreportedresults[30].WhenNaBrandCuSO4wereaddedtotheyellowAgcolloid,thetime-dependentSPRabsorptionspectraofthemixtureareshowninFig.4.TheSPRabsorptionspectraweremeasuredatatimeintervalof0.5s.Although121spectrawereobtainedin60s,only5typicalonesatdifferentmomentsareshowninFig.4.Theinsetsshowthetemporalabsorbanceofca.410nm( max)atatimeintervalof0.5s.Clearly,withtheincreaseofreactiontime,theSPRabsorbancedecreasedgradually.Afterthereaction,theyellowcolorofthemixturedisappeared.TheseresultsindicatedAgNPscouldreactwithCuSO4inthepresenceofNaBrandtheconcentrationofAgNPsdecreasedduringthereaction.

Subsequently,toascertainthereactionproductofAgNPs–NaBr–CuSO4,thereactionsystemwasevaporatedandprecipitatesoccurredinthereactedmixture.Thebrownpow-dersderivedfromtheprecipitateswereinvolvedintoXRDandXPSanalysis.InFig.5,XRDpatternoftheprecipitatefromtheAgNPs–NaBr–CuSO4systemisingoodagreementwithXRDpatternofpureCuBrandAgBr[31,32].Thepeaksat30.9 ,44.4 ,

188N.Lietal./JournalofPhotochemistryandPhotobiologyA:Chemistry

215 (2010) 185–190

Fig.4.Time-dependentUV–visabsorptionspectraduringthereactionbetweenCuSO4andAgNPsinthepresenceofNaBr.5selectedspectrawererecordedat2.5,5.0,10.0,18.0and32.0saftertheadditionofCuSO4inthemixingsolutionoftheAgcolloidwithNaBr.TheinsetsdisplaythecorrespondingtemporalSPRabsorptionatca.410nm(SPRabsorptionnaximumwavelength)atatimeintervalof0.5second.Conditions:1×10 3mol/LofCuSO4,0.01mol/LofNaBr,3×10 4mol/LofAg.

55.2 ,73.3 exactlycorrespondedtothepeaksof(200),(220),(222),(420)ofAgBr,respectively.Otherpeaksat27.3 ,45.1 ,53.5 exactlycorrespondedtothepeaksof(111),(220),(311)ofCuBr,respectively.TheresultsindicatedthatCuBrandAgBrwereareductionproductandanoxidationproduct,respectively.ItstronglysupportedthatAgNPscouldbeoxidizedbyCuSO4bytheaidofNaBr.Besides,Cu(I)compoundswereoneofmainproductsinthereaction.

Furthermore,Cu2pX-rayphotoelectronspectraofthereac-tionproduct,thebrownpowders,derivedfromtheprecipitates,areshowninFig.6(B).Allbindingenergies(BE)werecalibratedwithrespecttotheC1sBEat284.6eV.AscanbeseenfromFig.6,theCu2pspectrumoftheprecipitateswascurve- ttedintofourcomponentsat932.0,934.4,951.8,and954.3eV.Thecomponentsat932.0and951.8eVwereattributedtotheCu(I),whichisingoodagreementwithCu(I)2pXPSofpureCuBrasshowninFig.6(A).Thecomponentsat934.4and954.3eVwereattributedtoCu(II),beingingoodagreementwithCu(II)2pXPSofCu(II)inreportedresults[33].Additionally,thecomponentsat941.8and943.7eVinFig.6(B)belongtotheCu(II)3pspectrum.ThedataofXPSdemon-stratedthattheaverageatomicratio(Cu(I)/Cu(II))was1.6:1

on

Fig.5.(A)TypicalXRDpatternofCuBr.(B)XRDpatternoftheprecipitatefromtheCuSO4–AgNPs–NaBrreactionsystem.(C)TypicalXRDpatternof

AgBr.

Fig.6.X-rayphotoelectronspectraof(A)Cu2pofpureCuBrand(B)Cu2poftheprecipitatefromtheCuSO4–AgNPs–NaBrreactionsystem.

thesurfaceofprecipitates,indicatingthatCu(I)wasmainformofCu.

Therefore,thereactionbetweenCuSO4andAgNPsinthepres-enceofNaBrproducedCu(I)complexinits nalproducts.IthasbeenreportedthatCu(I)canreactwiththedissolvedoxygentoproducesuperoxide[34,35].TheCLoftheluminol/superoxidesys-tem,whichwasoneofthecommonCLsystems,hasbeenstudieddeeplyinthepast[36–38].ItisreasonabletosuggestthattheinjectionofluminoltotheCu(I)solutioncouldyieldCLemission.Subsequently,theCLreactionofCuBrwiththeluminolwasexam-ined.CuBrwasdissolvedintoaNaBrandHClsolution.AsshowninFig.7,themixtureofothercocomitants(AgNPs–AgNO3–NaBr)wasexployedinCLtestandnoconsiderableCLsignalappearedwiththeinjectionofluminol.However,whenluminolwasinjectedintotheCuBrsolution,aconsiderableCLemissionwasobserved.Con-sideringAgNPsorAg(I)complexmightcoexistwithCu(I)complex,AgNPsorAgNO3wasalsoaddedintotheCuBrsolutionfortheCLtests.InthepresenceofAgNO3orAgNPs,avisibleincreasein

the

Fig.7.Chemiluminescencekineticcurves.(a)Controlsolution:themixtureofAgNPs–AgNO3–NaBr.(b)CLoftheCu(I)complexsolutionwithluminol(dotline).Coexistreagents,AgNO3(c)(shotline)orAgNPs(d)(shotdotline)wereaddedintotheCuBrsolutionforCLtests.TheinsetsdisplaystheinhibitioneffectofSODontheCu(I)–luminolCL.Conditions:1×10 4mol/LCuBrin0.01mol/LNaBrand0.1mol/LHClsolution,3×10 4mol/LAg,1×10 5mol/LAgNO3,1×10 4mol/Lluminolin0.1mol/Lcarbonatebuffers(pH10.0).Controlsolution:3×10 4mol/LAg,1×10 5mol/LAgNO3,0.01mol/LNaBr.

N.Lietal./JournalofPhotochemistryandPhotobiologyA:Chemistry215 (2010) 185–190189

CLintensityappearedafterinjectingluminolintotheCuBrsolu-tion.ItissupportedthattheCu(I)complexsolutioncouldreactedwithluminoltogenerateCLemissionandthecoexistedAgNO3orAgNPscouldenhancetheCLintensity.Then,SODwasusedtodeter-minewhetherasuperoxideanionwasinvolvedintheCLreactionoftheCu(I)complexsolutionandluminol.SODwasaddedintotheCuBrsolutionbeforeinjectionofluminolandtheCLwasdecreasedwiththeincreaseoftheSODconcentration.TheseresultsrevealedthatasuperoxideanionwasinvolvedintheCLreaction.Onthisbasis,itwassuggestedthattheluminolCLinducedbyAgNPsinthepresenceofNaBrderivedfromCu(I)complex,oneofprod-uctsinthereactionofAgNPs–NaBr–CuSO4,whichreactedwiththedissolvedoxygentogeneratethesuperoxideanion;thenthesuper-oxideanionreactedwithluminoltoproduceCL.TheCLprocessmayproceedasfollows:

Cu(II)+AgNPs(0) Br→

Cu(I)+Ag(I)(1)Cu(I)+O2→O2 +Cu(II)(2)LH +O2 →AP2 +N2(3)AP2 →AP2 +hv

(4)

Basedontheproposedmechanism,Cu(I)complexisakeyintermediateproductintheCLreaction.Br isconsideredasanucleophileandtherearetwoimportantfunctionsintheluminolCL.

TheonefunctionofBr istoenhanceredoxcapabilityofAgNPs,whichreducesCu(II)togenerateCu(I)complex.Increasingthecon-centrationoftheBr-couldcausemoreAgBrtoprecipitate.BasedonNernst’sequation,precipitationofAg+withhalidescoulddecreasethereductionpotentialofAg(0).Therefore,thetendencyoftheoxi-dationofAgNPsbyCu(II)andthegenerationofCu(I)increasewiththeNaBrconcentration.

ThedismutationofCu(I)toCu(0)andCu(II)inwateriswell-known.Withoutnucleophiles,Cu(I)wasconsumedinthedis-mutationreactionbeforereactedwithoxygensothattheCLofCu(I)withluminolwouldbeterminated.Fortunately,Br intheCLreac-tionwasef cientligandstobindtoCu(I)andtopreventCu(I)fromdismutation:

Cu++2Br →CuBr2 (5) Kc=

[CuBr2][Cu+][Br ]2

=105.89

(6)

[CuBr 2][Br ]2

[Cu]

=105.89×=105.89×10 4=101.89=77.6

(7)

whereKcisthesteadyconstantofCuBr2 .With0.01mol/LBr inasolution,theconcentrationratio[CuBr2 ]/[Cu+]isabout78.Accordingly,Cu(I)mightbeintheformofCuBr2 intheCLreactionsystembythevirtueofBr .Br intheluminolCLisef cientligandsforCu(I)sothatCu(I)couldbeaccumulatedinthesolutionfortheCLreaction,whichisanotherfunctionofBr .3.3.TherespondofothernucleophilestotheCL

Sinceothernucleophiles,suchasCl ,I ,andthiosulfatearealsoef cientligandsforCu(I),itisreasonabletodeducethatthesimilarreactionscouldgeneratetheCLinthepresenceofthesenucle-ophiles.However,whenNaClsubstitutedforNaBr,AgNPscouldnotinducetheluminolCLunderthesamereactionconditions.ItisknownthatthesolubilityproductsofAgCl(Ksp,AgCl=1.8×10 10)aremuchlargerthanthatofAgBr(Ksp,AgBr=5.0×10 13).Accord-ingly,thereducingabilityofAgNPsinthepresenceofNaClisweakerthanthatinthepresenceofNaBratthesameconcentration.Dur-ingtheexperiments,whenCuSO4wasaddedintoAgcolloidinthe

presenceofNaBr,thecolorofAgcolloidfadedquicklyandobvi-ously.ButinthepresenceofNaCl,therewasnovisiblebleaching

whenCuSO4wasaddedintotheyellowAgcolloidinashorttime.Wededucedtheinef cientofCl wasrelativetotheunproductivereactionsothattheCLwastooweaktobedetected.Inordertoaccu-mulateenoughCu(I)inthereactionbetweenCuSO4andAgNPsinthepresenceofNaClfortheCLreaction,luminolwasinjectedintothemixtureafterwaitingaquitelongertime.Thewaitingtimewasinvestigated.Morethan600safterCuSO4mixedwithAgNPsinthepresenceofNaCl,aconsiderableCLappearedwheninjectinglumi-nolintothemixture.Furthermore,NaIwasalsoinvestigatedintheCLreactioninsteadofNaBrandtheCLemissioncouldbeobserved.Asamatteroffact,I-ismoreef cientligandforCu(I)thanBr .How-ever,I couldreactwithCu2+toyieldCuIandI2.AlthoughCuIcouldleadtotheCLemissionofluminol,I2–luminolsystemisanotherwell-knownCLsystem.Inotherwords,Cu2+–I –luminol–Agsys-temwithadifferentmechanismfromothersystemscouldnotbetakenasanexamplefordiscussion.Accordingly,Br waschosenasthemainnucleophileinthiswork.Besides,thiosulfate,agoodligandforCu(I)intheaqueoussolution,wasusedasanucleophiletobetestedintheCLreaction.TheresultswiththiosulfateweresimilartoNaBr.AgcolloidfadedwhenmixingwiththiosulfateandCuSO4andaconsiderableCLemissionwasgeneratedbyfurtherinjectingluminol.4.Conclusions

InthepresenceofnucleophilesandCu2+,AgNPscouldinitiateluminolCL.TheAgNPs–NaBr–CuSO4–luminolreactionwaschosenasamodelsystemtostudytheCLprocess.TheCLmechanismislikelyduetothefactthatAgNPsbytheaidofNaBrcouldbeoxidizedbyCuSO4toproduceCu(I)complex;Cu(I)complexreactedwiththedissolvedoxygentogeneratethesuperoxideanion;thenthesuper-oxideanionreactedwithluminoltoproduceCL.Cu(I)complexisakeyintermediateproductintheCLreaction.Inthepresenceofothernucleophiles,suchasCl ,I andthiosulfate,AgNPscouldalsoreactwithCu(II)toproduceCu(I)andnucleophilescouldalsobindtoCu(I)preventingthemfromdismutation,whichcouldinducetheluminolCL.

Acknowledgements

The nancialsupportoftheresearchbytheNationalNaturalSci-enceFoundationofPRChina(GrantNos.20573101and20625517)andtheOverseasOutstandingYoungScientistProgramofChinaAcademyofSciencesaregratefullyacknowledged.References

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